SULAYMANIYAH, Iraq, July 25 -- An opposition party promising to break the grip of the two ruling parties in northern Iraq's autonomous Kurdish region made a surprisingly strong showing in elections Saturday that appeared certain to shift the region's political arithmetic, opposition and government officials said.

Officials stressed that the results were preliminary and that a more accurate picture might not emerge until Sunday or later. Final results, to be announced in Baghdad, could take a week. But as early returns came in late Saturday, dramatically different moods descended over the opposing camps' headquarters.

Opposition officials said they were outpolling the ruling parties 2 to 1 in some parts of the crucial battlefield of Sulaymaniyah. Government and opposition officials said the opposition also did unexpectedly well in the other key province of Irbil.

"Early results from some of the locations in Sulaymaniyah and some in Irbil are not good," said Barham Salih, a veteran politician who was the ruling parties' candidate for prime minister of the Kurdish region. "It's anxious moments. We will see."

Overall, Salih said, the early results were "surprising."

Voters were choosing a president and a 111-member parliament for the Kurdish region, which has a remarkable degree of independence from Baghdad and is widely seen as a success story in an otherwise turbulent country. Polls were kept open an extra hour across the region's three provinces, and the electoral commission, citing preliminary figures at a news conference in Irbil, said turnout was 78.5 percent.

More than 500 candidates were running for parliamentary seats. Massoud Barzani, the incumbent president and head of the Kurdistan Democratic Party, one of the two parties that have ruled the region for a generation, faced five challengers. In parliament, Barzani's list of candidates had agreed on a joint slate with the other ruling party, the Patriotic Union of Kurdistan, led by Iraqi President Jalal Talabani.

Hardly anyone here Saturday expected Barzani to lose the presidency, and the joint list still seemed likely to capture a majority in parliament. But it faced a spirited challenge by dissidents from Talabani's party running as the Change List, particularly in Sulaymaniyah. Many observers were watching how many seats Change won as a barometer of the ruling parties' staying power and the discontent they must now reckon with. More than 15 seats would be considered a victory for the opposition, analysts and officials said.

Late Saturday, the challengers appeared to be polling far better than that, in what was shaping up as a protest vote that crossed lines of class, party and clan.

"It will certainly change the political landscape in Kurdistan," said Hiwa Osman, an editor and former spokesman for Talabani. "However many seats Change gets, Kurdish politics have changed. It heralds a new era that's going to dictate its own logic."

Nosherwan Mustafa, a founder of Talabani's party and now the head of the Change List, added: "We don't want to change just the faces and the persons. We want to change the political system. We want to separate the political parties from public life."

The two parties have run the Kurdish region since former president Saddam Hussein withdrew Iraqi troops after the 1991 Persian Gulf War and a Kurdish uprising. They fought a bloody civil war in the decade that followed, reconciled and have ruled the region jointly since the U.S.-led invasion of 2003, despite lingering tensions.

Over those years, the parties hewed to a formula that exchanged political plurality for stability, bringing prosperity to the Kurdish region and turning Irbil, its capital, into a Middle Eastern boomtown. Success, though, came at a cost. Under a relentless sun Saturday, many voters accused the parties of an almost complete lack of accountability in the control they exerted over most aspects of life, from appointments to lowly government jobs to multibillion-dollar deals.

Corruption was rife, they said, and jobs were few.

The disenchantment has overshadowed the growing battle with the federal government in Baghdad over the disputed boundaries between Kurdish and Arab Iraq and a law on sharing oil revenue and management of the country's sprawling reserves.